This invention relates to chemical vapor deposition (CVD) equipment and, in particular, to apparatus for directly measuring the temperature of a semiconductor wafer.
In operating CVD equipment, it is known that the deposition of material is very dependent upon the temperature at which the chemical reaction is carried out. Sensing this temperature in a reliable manner sufficient for closed loop temperature control has been a continuing problem.
In the past, the temperature within the CVD reaction chamber has been sensed by probes at various locations within the chamber. A problem with these techniques is that the temperature of the wafer is what largely determines deposition parameters, e.g. rate, film properties, and uniformity, not the temperature elsewhere in the chamber.
A problem with placing a probe directly on the front side of the wafer is that it causes a shadow in the film grown on the wafer and may contaminate the film or the wafer. Further, the probe is exposed to the chemicals within the chamber, which tend to deteriorate the probe. Where the wafer is heated via a susceptor, e.g. a graphite block on which it rests, the temperature probe is often placed in the block. This still provides only an approximate temperature reading since the temperature of the susceptor is being measured, not the wafer itself.
In one particular CVD reactor of the prior art, as disclosed in U.S. Pat. No. 4,632,056, the wafer is directly and rapidly heated by a plurality of lamps, i.e. there is no susceptor. The wafer rests on a quartz window and the heat lamps are on the opposite side of the window. Temperature is measured by modeling the wafer with a ceramic disc, the temperature of which is measured directly.
Despite these various attempts, the problem remains of not knowing the actual temperature of the wafer from contact by a probe, e.g. a thermocouple. This is particularly true where the wafer is heated and cooled relatively quickly. The problem is more difficult in systems which directly heat the wafer since the probe cannot interfere with heating the wafer or be subjected to the gases within the chamber. Contacting the backside of the wafer may interfere with heating and/or require machining the quartz window. Quartz is difficult to work with and does not seal well with most ceramics at the temperatures to which it will be subjected in CVD equipment.
In view of the foregoing, it is therefore an object of the present invention to provide means for directly measuring the temperature of a rapidly heated wafer without disturbing the uniform heating thereof.
Another object of the present invention is to provide improved CVD equipment having temperature control based upon direct measurement of wafer temperature.
A further object of the present invention is to provide apparatus for reliably making thermal contact with a semiconductor wafer.